Field of the Invention
The present invention relates to a current detection circuit which monitors a current, and more specifically to a circuit which measures a voltage across a resistor inserted into a power supply line by a differential amplifier and outputs a measurement result.
Background Art
Monitoring a current in a power supply line is important as means for detecting abnormalities such as a short circuit of the power supply line to a ground line, disconnection of parts connected to the power supply line, etc.
A circuit diagram of a related art current detection circuit is illustrated in FIG. 3. The related art current detection circuit is equipped with a resistor 301 for converting a current flowing through a power supply line 300 into a voltage, and a differential amplifier 320 for amplifying a difference in voltage between both ends of the resistor 301. The differential amplifier 320 is comprised of resistors 306 and 307, PMOS transistors 308 and 309, current sources 310 and 311, a non-inversion input terminal 303, an inversion input terminal 302, and an output terminal 304.
Such a current detection circuit as described above is operated in the following manner and has a function of monitoring the current.
When the current in the direction of an arrow flows through the resistor 301 (IS=+ current), it is shown that since the non-inversion input terminal 303 is at a higher voltage than the inversion input terminal 302, and a source-gate voltage of the PMOS transistor 309 is larger than a source-gate electrode of the PMOS transistor 308, the voltage of the output terminal 304 is raised so that a + current flows.
When the current flows through the resistor 301 in the direction opposite to the arrow (IS=− current), it is shown that since the inversion input terminal 302 is at a higher voltage than the non-inversion input terminal 303, and the source-gate voltage of the PMOS transistor 309 is smaller than the source-gate voltage of the PMOS transistor 308, the voltage of the output terminal 304 is lowered so that a − current flows.
When the resistors 306 and 307 are equal in resistance value, the PMOS transistors 308 and 309 are equal in characteristics, and the current sources 310 and 311 are equal in current value, a threshold value at which the voltage of the output terminal 304 is inverted becomes a condition (IS=0 mA) that no current flows through the resistor 301.
[Patent Document 1] U.S. Pat. No. 5,969,574, Specification